One night in July 2003, a little before midnight, a plainclothes N.Y.P.D. detective, investigating a series of car thefts in upper Manhattan, followed a suspicious-looking young man with long, stringy hair and a nose ring into the A.T.M. lobby of a bank. Pretending to use one of the machines, the detective watched as the man pulled a debit card from his pocket and withdrew hundreds of dollars in cash. Then he pulled out another card and did the same thing. Then another, and another. The guy wasn't stealing cars, but the detective figured he was stealing something.